1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using an electrophotographic printing method, and more particularly, to an image forming apparatus using exposure technology for exposing a photosensitive body to light beams.
2. Description of the Related Art
[Laser Driving with PWM]
Conventionally, an image forming apparatus using an electrophotographic printing method has been known. The image forming apparatus of this type forms an electrostatic latent image on a photosensitive body by exposing the photosensitive body to laser light from a laser that is driven on and off according to a PWM signal (a pulse width modulation signal) pulse-width-modulated for each pixel based on image data. The electrostatic latent image is developed as a visible image by a developer such as toner, and transferred to recording paper.
The PWM signal is generated by a pulse width modulation circuit (a PWM circuit) as shown in FIG. 10. The PWM circuit outputs a pulse signal having a value corresponding to a value of the input image data (a density value), and outputs the pulse signal to a laser driver for driving the laser.
The PWM circuit has a D/A conversion circuit 401, a triangle wave generation circuit 402, a comparator 403, and an oscillator 404. The D/A conversion circuit 401 converts the input digital image data, for example, of 8-bit, into analog image data, and outputs the analog image data to the comparator 403. The triangle wave generation circuit 402 generates a triangle wave in the same cycle as a clock signal from the oscillator 404, and outputs the triangle wave to the comparator 403 as a reference wave.
The comparator 403 compares the D/A-converted image data (a D/A output signal) with the triangle wave as the reference wave, and outputs to the laser driver (not shown) the PWM signal that becomes high level only when the image data is larger. For example, when the image data is expressed as 00h to FFh, and when the magnitude relation between the image data and the triangle wave is as shown in the upper part of FIG. 11, the comparator 403 outputs the pulse width modulation signal (a PWM signal) shown in the lower part of FIG. 11.
In other words, in a period for one pixel denoted by a reference numeral 51, the image data is larger than the triangle wave throughout the period, and accordingly, the PWM signal becomes “high” throughout the period for this pixel. On the other hand, in a period for one pixel denoted by a reference numeral 52, the image data is smaller than the triangle wave throughout the period, and accordingly, the PWM signal becomes “low” throughout the period for this pixel. In a period for one pixel denoted by a reference numeral 53, the image data is smaller than the triangle wave in the beginning and the ending portions of the period and the image data is larger than the triangle wave in a central portion of the period, and accordingly, the PWM signal becomes “low” in the beginning and the ending portions and becomes “high” in the central portion.
The laser driver turns on (lights) the laser in a period when the input PWM signal is “high” and turns off (extinguishes) the laser in a period when the input PWM signal is “low”.
[Multiple Exposure Method]
More and more image forming apparatuses using an ROS (Raster Output Scanner) exposure device have made their exposure light sources into multi-beam, which is a technology that copes with increased speed and resolution of the apparatuses. This multi-beaming enables forming a plurality of scanning lines with one scan, thus enabling increased resolution and speed without increasing the number of revolutions of a polygon mirror.
However, the multi-beam leaves a streak in a scanning direction on an output image where the amount of light varies in each of the light beams. In a case where mirror surfaces of the polygon mirror vary in angle, scanning intervals vary to result in uneven streaks corresponding to the variety of the scanning intervals.
A single beam has narrower and inconspicuous scanning intervals and is thus less prone to cause an image defect, but the multi-beam has wider and conspicuous scanning intervals and thus has a tendency to cause the image defect. In a case where a surface-emitting laser made into the multi-beam is used, it is difficult to increase light amount compared with a laser of the single beam.
To solve such problems, a multiple exposure method has been proposed in which a plurality of light beams from different light sources are used to expose an identical pixel position of the photosensitive body for a plurality of times (see, Japanese Laid-Open Patent Publication (Kokai) No. 2002-264391 and Japanese Laid-Open Patent Publication (Kokai) No. 2004-109680).
In the meantime, an electric current equal to or more than a predetermined value (a threshold current) is needed to be applied to a laser for emitting laser light. Accordingly, where a pulse width of the PWM signal is narrow, laser emission may not be obtained depending on an emission characteristic of the laser due to a delay in emission. Therefore, in prior art, a bias electric current not exceeding the threshold current is previously applied to the laser. The application of this bias electric current enables reducing the amount of a switching electric current (a driving electric current by the PWM signal) up to reaching the threshold current, and enables the laser emission to be obtained even where the pulse width of the PWM signal is narrow.
However, the threshold current varies depending on a temperature, and for example, setting the bias electric current approximately the same as the threshold current may cause the laser to emit light by the bias electric current depending on the temperature. Accordingly, a set value of the bias electric current has to be made lower to some extent, and even where the bias electric current is applied, the pulse width of the PWM signal for laser emission, namely, a period of “high” (an ON period), is required to be made more than a certain width.
On the other hand, where the pulse width of the PWM signal is too wide in a period for one pixel, the laser is turned on throughout the period for the one pixel due to a delay in turning off even where there exists an OFF period of the laser within the period for the one pixel. This is caused by the laser driver failing to fully respond because a period in which the PWM signal is “low” is short within the period for the one pixel.
Therefore, where the laser is driven by the PWM signal, it causes a delay in emission and a delay in turning off. This defect is described based on FIG. 12. FIG. 12 is a figure showing a relationship between an image data value and light amount of laser emission of prior art. It should be noted that the image data value represents a density value and is proportional to a “high” duty of the PWM signal, namely, proportional to a value of the PWM signal in an electric current-supplied time.
In FIG. 12, in the electric current-supplied time where the image data value is in the range of “00h to 30h”, there is no laser emission, and the output light amount of the laser is constant. Then, in the electric current-supplied time where the image data value is in the range of “30h to B0h”, the relationship is linear between the image data value and the output light amount of the laser.
In the electric current-supplied time where the image data value is in the range of “B0h to D0h”, linearity is not lost between the image data value and the output light amount of the laser, but the inclination becomes rapidly large to make it difficult to precisely control the output light amount of the laser. In the electric current-supplied time where the image data value is in the range of “D0h to FFh”, the output light amount of the laser is saturated at the maximum amount of light.
To cope with this, a prior art image forming apparatus corrects the triangle wave as the reference wave and the D/A output signal so that the relationship between the image data value of “00h to FFh” and the output light amount of the laser is made linear as shown in the range of “30h to B0h” of FIG. 12.
That is, when the image data value is “00h”, by providing the triangle wave with an offset component, the actual pulse width of the PWM signal is set to be the pulse width corresponding to a case where the D/A output signal value of FIG. 11 is “30h”. On the other hand, when the image data value is “FFh”, by adjusting the D/A output signal value with respect to the image data value, the actual pulse width of the PWM signal is set to be the pulse width corresponding to a case where the D/A output signal value of FIG. 11 is “B0h”. Therefore, the relationship between the image data value and the output light amount of the laser is kept linear.
For example, in a case of outputting a line image and the like in which the image data values are “FFh” successively in a main scanning direction (an axial direction of the photosensitive body), the laser operates in intermittent light emission corresponding to the above-described pulse width “B0h” in each pixel period, thus causing a non-lighting period to occur. As shown in FIG. 13, this non-lighting period occurs at the beginning and the ending period of each pixel period, namely, occurs at periods continuing to a neighboring pixel. Thus, there has been a problematical point that a line becomes narrow or is interrupted and that the quality of a line image and a character image is thus deteriorated.